1. Field of the Invention
The present invention relates to an APF (Alternating-Phase-Focused) linear ion accelerator that accelerates an ion beam, such as a carbon beam or a proton beam, to obtain the ion beam of high energy.
2. Description of the Background Art
An APF linear ion accelerator includes an acceleration cavity in which a plurality of cylindrical electrodes called drift tubes (hereinafter referred to simply as drift tubes) are arranged along the linear path of an ion beam that is injected into the acceleration cavity, so that the lengths of the drift tubes are changed sinusoidally, in consonance with a predetermined cycle, in the direction in which the ion beam passes. This change in tube lengths is hereinafter called an oscillation having a predetermined cycle. Furthermore, gaps are formed between the drift tubes, while a radio frequency acceleration electric field is applied to the individual gaps. Thereafter, when an ion beam passes across one of the gaps (hereinafter referred to as acceleration gaps), the ion beam is accelerated by the radio frequency acceleration electric field applied to the gap, and simultaneously, a focusing force is applied to the ion beam in the transverse direction (which is perpendicular to the direction of travel of the beam, which is termed the vertical direction). When an ion beam has been accelerated and has attained a predetermined extraction energy by passing across a predetermined number of acceleration gaps, the ion beam is extracted from the linear ion accelerator as an extraction beam
(Non-patent Document 1) Y. Iwata, et.al., “Alternating-Phase-Focused Linac for an Injector for Medical Synchrotron,” Proceedings of EPAC 2004, Lucerne, Switzerland, p2631.